Nonlinear complex oxide (NLCO) thin film materials are a critical class of materials to enable low cost frequency agile voltage controlled tunable Radio Frequency/Microwave Frequency (RF/MW) devices. Technological systems applications for these devices include, but are not limited to, commercial wireless communication systems (cell phones), military hand held communications systems (software-defined radios), mobile electronic scanning antennas, phased array Radars and advanced Electronic Warfare systems etc. These application technologies demand materials which have their own specialized requirements and functions; hence NLCO materials such as strontium titanate (SrTiO3/ST), barium titanate (BaTiO3/BT), barium strontium titanate (Ba1-xSrxTiO3 or BST), and variations thereof are considered to be the prime candidate material systems for these device/system applications.
In order to be useful in practical commercial and military applications these NLCO based-thin films must meet several stringent material property requirements, namely; a high dielectric constant (enables device miniaturization and promotes wide tunability), low dielectric loss/high dielectric Quality Factor Q (maximizes signal intensity and transmission, ensures reduced operational power of the device), low leakage current (minimizes battery/power draw), low temperature coefficient of dielectric constant (maximizes temperate stability), high break down field strength (ensures extended device operating reliability), and a smooth defect free surface morphology to promote reliable thin film integration with top electrodes (minimizes conductor loss; hence minimizes signal attenuation). For NLCO thin films it is known that the material properties such as loss and leakage current have been improved by acceptor doping; material temperature stability has been demonstrated in compositionally stratified NLCO heterostructures and the break down strength has been extended via defect mitigation and improved film-electrode interfaces. Battery draw has been minimized by optimization of bottom electrode, i.e. smooth defect free bottom electrode-active thin film interface, and the reduction of the grain boundary area in the active film. However, the simultaneous attainment of a high dielectric constant and low dielectric loss has been mutually exclusive in NLCO thin films. At the same time NLCO thin films offer overall lower dielectric constants and tunability, with higher losses, compared to their bulk materials counterparts. Reasons for such degradation in the NLCO thin film material performance and the inability to simultaneously achieve a low loss and high dielectric constant has been attributed to film thermal strain, interfacial dead layers, and local polar regions near charged defects like oxygen vacancies. Among these, film thermal strain is considered the most critical problem that must be overcome in order to improve the material properties and create “device-ready” NLCO thin film materials.
In compositionally homogeneous NLCO thin films, internal stresses arise because of lattice mismatch between the film and substrate if the films are epitaxial, the difference in coefficients of thermal expansion (CTE) of the film and the substrate, self-strain (volume changes) of ferroelectric phase transition if the material is grown above the phase transformation temperature, and microstrains due to defects such as dislocations and vacancies. Since NLCO films are processed/crystallized at elevated temperatures (600-1100° C.) the CTE mismatch between the film and the substrate which determines the thermal strains becomes a significant parameter in limiting the dielectric response of these NLCO thin films. Thus, inventive strategies to reduce and/or mitigate the detrimental effects of thermal strains in NLCO thin films is paramount for realizing enhanced dielectric response and low loss in concert with one another to enable device miniaturization, wide tunability, enhanced signal intensity/transmission and reduced operational power (i.e., the inverse relationship between dielectric constant/tuning and dielectric Q must be overcome). Furthermore the requisite for device affordability requires that the NLCO thin films be deposited/grown by a technique that is main-stay to the semiconductor industry, i.e. semiconductor foundry friendly, and that these films be integrated with large area low cost IC-compatible device-relevant substrates, namely high resistivity silicon. Furthermore materials/device affordability is further achieved by meeting the demands mass production whereby the materials and associated processing science strategies required to reduce thermal strain must be complementary metal oxide semiconductor (CMOS) compatible and semiconductor foundry friendly. As such RF/MW tunable devices with enhanced dielectric response, low loss/high dielectric Q in combination with high dielectric constant/high tunability that satisfy the economy of scale and affordability associated with the semiconductor industry are desirable.